An infrared imaging apparatus using an infrared image sensor is known as an imaging apparatus. An infrared imaging apparatus using an infrared image sensor has a characteristic of capable of measuring temperature contactlessly, and thus is used for applications, such as security, medical care, maintenance, research and development, military affairs, and the like. For example, an infrared imaging apparatus is used for contactlessly measuring the temperature of passengers at an airport and to extract patients of infectious diseases. Also, an infrared imaging apparatus is sometimes used for a night vision device. In the following, a description will be given of an example of an infrared imaging apparatus using an infrared imaging device. However, the described technique is not limited to this case.
FIG. 1 is a diagram illustrating an example of the configuration of an infrared imaging apparatus 1 using an infrared image sensor 14. The infrared imaging apparatus 1 includes an imaging unit 10 and a signal processing circuit 18 that performs signal processing on the infrared imaging signal output from the imaging unit 10. The imaging unit 10 includes a lens 11 and an infrared image sensor 14. The signal processing circuit 18 includes a sensitivity correction calculation circuit 16 and an imaging circuit 15.
FIG. 2 is a diagram illustrating an example of a configuration of the infrared image sensor 14. The infrared image sensor 14 detects the infrared light emitted from an observation object in accordance with the surface temperature of the observation object by a plurality of sensor elements disposed in a two-dimensional array. The infrared image sensor 14 then outputs a sensor output signal in order to generate a thermal image (thermographic image) indicating the distribution of the surface temperature of the observation object.
The infrared image sensor 14 includes a sensor array 12 and a complementary metal oxide semiconductor (CMOS) reading circuit 13. The CMOS reading circuit 13 is disposed on a substrate. The corresponding electrodes of the sensor array 12 and the CMOS reading circuit 13 are coupled by bumps 17 that are made of indium. The sensor array 12 includes a plurality of sensor elements (pixels) that are disposed in a two-dimensional array. Each sensor element of the sensor array 12 is a photoconductive element having a characteristic of changing the resistance value in accordance with the amount of incident infrared light.
The lens 11 (refer to FIG. 1) projects the infrared light emitted from the observation object onto the sensor array 12. Each sensor element of the sensor array 12 generates a photoelectric current in accordance with the amount of the incident infrared light that is projected. Thereby, the infrared light is converted to an electronic signal. The electronic signal is multiplexed by the CMOS reading circuit 13 and then is output to the sensitivity correction calculation circuit 16 of the signal processing circuit 18. The imaging circuit 15 performs format conversion on the infrared imaging signal after the sensitivity correction processing by the sensitivity correction calculation circuit 16 into an image signal for generating a thermal image. A display monitor not illustrated in FIG. 1 displays a thermal image based on the image signal output from the imaging circuit 15.
FIG. 3 is a diagram illustrating an example of a configuration of the CMOS reading circuit 13. The CMOS reading circuit 13 includes a plurality of pixel circuits 21 and a scan circuit 25.
The scan circuit 25 includes a plurality of scan lines 27 that extend in parallel in the horizontal direction (row direction), a plurality of vertical bus lines 28 that extend in parallel in the vertical direction (column direction), a vertical scanning shift register 22, and a horizontal scanning shift register 23.
The pixel circuits 21 are disposed in a matrix state correspondingly to the individual intersecting units of the plurality of scan lines 27 and the plurality of vertical bus lines 28. A sensor element 24 in a pixel circuit 21 indicates a sensor element (cell) disposed in the sensor array 12 (refer to FIG. 2), which is a photosensitive unit of the infrared image sensor 14. The pixel circuit 21 is disposed for each of the plurality of sensor elements 24.
In the pixel circuit 21, a reset signal RS is applied to a transistor 36 for a reset gate, and thus the transistor 36 becomes conductive so that a storage capacitor 41 is charged at a predetermined value. After the application of the reset signal RS is stopped, an integration signal IG-T2 is applied to a transistor 35 for an input gate for a certain period of time, a current corresponding to the infrared light intensity flows through the sensor element 24, and thus the voltage of the storage capacitor 41 becomes a voltage corresponding to the infrared light intensity. Next, a transistor 37 for sample-and-hold reset becomes conductive in response to a reset signal SHRS, and thus the voltage level of a sample-and-hold capacitor 42 is reset to a predetermined value. Next, sample-and-hold signals SH and /SH are applied to a transfer gate 38, and thus the voltage of the storage capacitor 41 is transferred to the sample-and-hold capacitor 42 and held. The sample-and-hold signal /SH is the inverted signal of the sample-and-hold signal SH. Such an operation is individually performed in the plurality of pixel circuits 21 at the same time, and thus a voltage corresponding to the infrared light intensity of each sensor element 24 is held in each sample-and-hold capacitor 42.
The vertical scanning shift register 22 outputs a scan pulse V-Sel that selects a plurality of scan lines 27 one by one in sequence. A transistor 32 whose gate is coupled to a scan line 27 to which the scan pulse V-Sel is output becomes conductive in accordance with the scan pulse V-Sel. The voltage held by the sample-and-hold capacitor 42 of the pixel circuit 21 coupled to the conductive transistor 32 via the transistor 31 is individually output to the corresponding vertical bus line 28 via the transistor 31 and the transistor 32.
The horizontal scanning shift register 23 applies a reading pulse H-Sel to a transistor 33 in sequence. In response to the reading pulse H-Sel, the voltage of the vertical bus line 28 is output to a reading line 26, and an image signal voltage Vpxl arises. The image signal voltage Vpxl is out from a final output stage amplifier 29 as an analog output signal Vout in sequence.
When the output of the voltage of all the vertical bus lines 28 is complete, the vertical scanning shift register 22 applies the scan pulse V-Sel to the next scan line 27. After that, the above-described operations are repeated, and the signals of all the sensor elements 24 that are two-dimensionally disposed are multiplexed and output on one output line. A transistor 34 becomes conductive in response to a signal VRS and resets the reading line 26 to a ground level.
A timing control signal that operates the CMOS reading circuit 13, such as the reset signal RS, or the like is given from the timing generator 20.
However, if there is a charge trap on the gate electrode interface of a CMOS transistor, the channel potential of the transistor gets out of order by the existence of an electron that comes in and out from the trap. FIG. 4 is a diagram illustrating a part of a configuration of the pixel circuit. When the channel potential of the transistor 35 for an input gate gets out of order due to a charge trap, even if the voltage given to the gate of the transistor 35 from the outside is kept as a fixed voltage, the gate-to-source voltage of the transistor 35 changes, and thus the source potential of the transistor 35 changes. Changes in the source potential of the transistor 35 cause fluctuations of the bias voltage applied to the both ends of the sensor element 24, which brings about fluctuations of the photoelectric current that occurs in the sensor element 24. Accordingly, the amount of charge stored in the storage capacitor 41 and transferred to the sample-and-hold capacitor 42 fluctuates. Accordingly, the pixel output voltage read from the pixel circuit 21 fluctuates with time in accordance with a temporal change of the trap state of the transistor 35 in the input gate unit.
The fluctuations of the pixel output voltage often appear as binary fluctuations having a relatively large amplitude as illustrated in FIG. 5, and are referred to as random telegraph noise. FIG. 5 is a diagram illustrating an example of fluctuations of a pixel output voltage. In FIG. 5, a pixel output voltage for a certain pixel circuit is expressed by continuous data that is arranged continuously on the time axis. If fluctuations of the pixel output voltage for a certain pixel continues as illustrated in FIG. 5, the luminance corresponding to the pixel changes on the display monitor with time, and thus an image on the display monitor might be disordered.
The following is a reference document.    [Document 1] Japanese Laid-open Patent Publication No. 2011-142558.